The present invention relates to a classification gauge generally of a type disclosed in prior U.S. Pat. No. 3,464,119, issued Sept. 2, 1969 to the inventor of the present classification gauge. The classification gauge of the present invention, like the earlier classification gauge, relates to the determination of a relative effective open area in the various stages of turbojet engines. The classification gauge of the present invention is particularly directed toward such determinations in connection with vane clusters including a plurality of guide vanes, turbine blades and/or other similar structures.
Substantial background information is set forth by the above-noted patent and is not included herein since it is not believed essential to an understanding of the present invention. Generally, it is believed sufficient to understand that a certain effective open area between adjacent guide vanes for permitting the passage of gas therethrough, commonly referred to as "throat area", must be maintained or at least known with precision. The known throat area of a turbojet engine is the desired open area required to allow the engine to perform properly. As was discussed in substantial detail within the above-noted patent, the total open area or throat area in each row of guide vanes is very important. In an average size turbojet engine, it must not vary more than approximately 1 square inch from the determined parameters. If the area is too small, "choke-off" may occur and the desired quantity of fluid or gas cannot pass through that stage.
The throat area of a vane cluster can change during use because of distortions caused by heat, the force of the gases passing therethrough and erosion due to use. Accordingly, it may be necessary to monitor the effective throat area during use as well as upon initial assembly.
The classification gauge described by the above-noted patent is particularly adapted to determine the effective throat area and therefore the proper classification for individual guide vanes. That prior classification gauge functions generally to accurately position a trailing edge of the airfoil within a precision jig and then to arrange a spaced-apart portion of the airfoil in a position determined by other parameters including the efffective total length for the guide vane. In this manner, the guide vane is accurately positioned in an angular arrangement with an indicating guide then being effective to measure the relative angular position of a mounting flange for the guide vane.
The present classification gauge for vane clusters performs a generally similar function in connection with a plurality of such guide vanes or airfoils secured together in a fixed relationship suitable for mounting in a turbojet engine stage, for example.
The term "classification" as employed here refers to an arbitrary system used, for example, by airplane engine manufacturers for constructing or selecting the proper throat area for a particular stage of a turbojet engine. Each classification "number" represents a specific throat area.